This invention relates to seismic isolation bearings for buildings, bridges and other structures; specifically to ball bearings with nonlinear, gravity recentering capacity.
It is common to mount structures on seismic base isolators for safety and economical reasons.
The most frequently used isolators utilize rubber for motion decoupling and bearing recentering. Bearing recentering is shortly called restoring. Bearing's period, size and bearing capacity is limited by the rubber's buckling and roll over state conditions. Practical rubber bearings have around 2 second period; 50% shear strain; and 1,000 psi (lb/in/in) bearing capacity. (Shear strain is the shear displacement to rubber height ratio.) These parameters are not suitable for near fault located seismic isolators. (Near fault is less than 15 miles close to an active seismic fault. Seismic faults are common earthquake sources. Faults, like rivers, criss-crossing several big cities.) Rubber bearings can easily accommodate the 0.02 radian, usually required bearing rotation. Rubber bearings are made of high damping rubber or supplemented by dampers. That is to reduce bearing displacement.
Friction slider bearings were proposed to expand these limitations to about 4 second period and 3500 psi bearing pressure. That is the capacity of a stainless steel and teflon lined steel plate bearing assembly, called teflon bearing. Shear strain is undefined for friction bearings. Instead, the bearing displacement is considered. Teflon bearings are made to 2 to 6 feet displacement, often required at near fault installations. Unfortunately, teflon bearings need rubber plate bed to accommodate bearing rotation. Worst of all, sliding bearings have no restoring at all, thus tend to displace one way only. Restorer need to be supplemented to reduce bearing displacement. The use of rubber plate drops bearing capacity to 1,000 psi.
A recently proposed friction pendulum bearing further expanded the mentioned limitations, by adding gravity restoring. Its friction surface is polymer-matrix coated, mounted on an articulated slider. That articulation allows for bearing rotation accommodation. Spherical sliding surfaces provide proportional gravity restoring. (Proportional restoring has a rubber bearing too. It means linear bearing force-displacement relationship.) Friction pendulum bearings may have 2 to 4 second period, 2 to 4 feet displacement and 10,000 psi bearing pressure on the polymer. Unfortunately, the average bearing pressure remained even below 1000 psi. The constant period is an other draw back. Seismic isolators preferably need transitive period to avoid resonance. Flat friction sliders has transitive period. Transitivity means displacement dependence.
U.S Pat. No. 4,644,714 describes a friction pendulum bearing. Its mechanism mimics a pendulum having gravity restoring. U.S. Pat. No. 4,726,161 (Feb. 23, 1988) describes a rolling bearing with gravity restoring. This utilizes a spring loaded telescoping mast with a housed, lubricated ball bearing rolling in a spherical dish. Also a structural framing and a stabilizator. Due to its limited size and need for maintenance, its use is limited to museum object isolation.
It appears, progress fulfilled needs, but not yet. Ideally bearings should exclude the possibility of overstrain and overload. That means constant restoring and damping forces. Both to be independent of displacement. Constant (nonlinear) bearing forces means physical limitations. Seismic force on structures should be limited by physical means to a safe level. Elimination of rubber, teflon and polymer would be beneficial. That would alleviate the need for fire and weather protection. Also would enhance bearing longevity, alleviating the need for bearing replacement due to ageing. Added displacement limiting capacity would be a natural safety feature. At last, bearing pressures could be increased up to the steet's yield strength to further enhance economy.
It is therefore the main object of this invention is to provide an isolation bearing with constant restoring and damping forces.
Other objects are to provide a bearing with:
displacement independent (nonlinear) restoring; PA1 possibility of proportional restoring; PA1 steel or metal parts only; PA1 transitive period; PA1 very high contact pressure; PA1 high displacement capacity; PA1 capacity to accommodate high bearing rotation; and PA1 displacement limitation capability.